Specific instances of the conduct of a witness, for the purpose of attacking or supporting the witness' character for truthfulness, other than conviction of crime as provided in rule 609, may not be proved by extrinsic evidence. They may, however, in the discretion of the court, if probative of truthfulness or untruthfulness, be inquired into on cross-examination of the witness (1) concerning the witness' character for truthfulness or untruthfulness, or (2) concerning the character for truthfulness or untruthfulness of another witness as to which character the witness being cross-examined has testified.

As the recent opinion of the Eighth Circuit in United States v. Alston, 2010 WL 4923288 (8th Cir. 2010), makes clear, however, it is important to note that cross-examination under Rule 608(b) is still subject to the balancing test set forth in Federal Rule of Evidence 403, which provides that

Although relevant, evidence may be excluded if its probative value is substantially outweighed by the danger of unfair prejudice, confusion of the issues, or misleading the jury, or by considerations of undue delay, waste of time, or needless presentation of cumulative evidence.

In Alston, Donnell Alston was convicted on each count of a two-count indictment for (1) knowingly possessing with intent to distribute more than 50 grams of cocaine base; and (2) knowingly possessing with intent to distribute cocaine. At trial, Officer Greg Napier testified that after Alston was arrested, he

advised Alston of his Miranda rights and interviewed him. Alston admitted that the drugs and paraphernalia in the hotel room were his and that he had been distributing them. Detective Napier did not make an audio or video recording of Alston's confession but instead took notes of the statement and wrote a report.

Alston's theory of the case was that Napier created this confession out of cloth. The district court, however, precluded him from cross-examining Napier about an incident in which Napier allegedly challenged a prisoner to a fight and then lied about it afterward.

In rejecting Alston's appeal, the court found that this proposed cross-examination, while potentially valid under Rule 608(b), violated Rule 403 because

Detective Napier's testimony relating Alston's confession was certainly important. Yet, while "confessions have profound impact on the jury,"..., such that a police officer's description of a confession is correctly subjected to intense cross-examination, Alston's confession was not the only evidence linking Alston to room 416 and the drugs. As described at length above, the other evidence in this case circumscribes the importance of Detective Napier's testimony. Additionally, the proffered hearsay evidence is not all that probative of Detective Napier's character for truthfulness, even if it is true that Detective Napier lied to his superiors (a finding we do not make). It is evidence of an isolated event of a different character from the one at issue here. There, Detective Napier allegedly lied to protect himself from punishment. Here, Detective Napier essentially allegedly invented a complex confession to secure a drug conviction. The difference in motive is clear, and that difference lowers the probative value of the evidence....

In addition to having only limited probative value, the proffered cross-examination would have created a danger of unfair prejudice. As the exhibits to the Government's motion show, Detective Napier was found to have engaged only in ridiculing or taunting a prisoner and was reinstated. As a sister circuit has recognized, when the previous allegations of misconduct leveled against a witness resulted in no sanctions or sanctions completely unrelated to the witness' character for truthfulness, the danger is great that a jury will infer more from the previous investigation than is fairly inferable....To have allowed the cross-examination would have resulted in the kind of mini-trial on a peripherally related matter that the Rule is designed to prevent.